Three element diaphragm damper for fuel pump

ABSTRACT

A damper assembly for a fuel pump includes at least one diaphragm assembly formed by joining two metal diaphragms to respective two sides of an imperforate central plate, thereby creating a pair of closely spaced diaphragms, each acting upon its own gas volume. Preferably, the diaphragm assembly has (a) a rigid, relatively thick circular or polygonal central plate, (b) a first circular diaphragm having a rim portion sealingly secured as by welding to the plate and a relatively thin, flexible, convex portion projecting from one side of the plate and defining a first pressurized gas volume, and (c) a second circular diaphragm having a rim portion sealingly secured as by welding to the plate and a relatively thin, flexible, convex portion projecting from the other side of the plate and defining an independent second pressurized gas volume.

BACKGROUND

The present invention relates to high pressure fuel supply pumps and inparticular to damping pressure pulses arising from the reciprocation ofa pumping piston that pressurizes fuel for discharge from such pump.

An increasing number of fuel systems for on-road vehicles are adoptingthe so called “common-rail” configuration, whereby a fuel pump maintainsa reservoir or “rail” at a pressure at or above that required for fuelinjection, and a plurality of injectors are in fluid communication withthe rail via respective injection valves that are electronicallyactuated, thereby controlling the timing and quantity of fuel deliveredfrom the rail to each cylinder of the engine. The pumps typicallycontain at least one pumping piston that is reciprocated by a cammedconnection to the engine drive shaft. Common rail systems no longerrequire direct synchronization of a plurality of pumping pistons orrotating hydraulic head, with the injection events. Instead, the pumpneed only maintain the rail at the desired pressure. As a consequence,designers now favor single piston pumps for a wide variety of operatingregimes.

The high pressure pump is typically fed from a low pressure feed lineentering the charging line of the pump. Due to the high rate of pistonreciprocation of a single piston to produce a sufficient number ofdiscrete quantities of high pressure fuel discharged into the rail, thecharging system of the pump can experience significant back pressurepulses which produce unwanted noise and wear on the pump internals.Although techniques are known for back pressure relief to a low pressuresink or auxiliary accumulator, vehicle manufacturers seeking to conservespace in the engine compartment and save cost, are increasinglyrequiring even single piston fuel pumps to have internal damping, i.e.,without flow to a low pressure fuel reservoir or fuel accumulator.

U.S. Publication 2008/0175735 “Inlet Pressure Attenuator for SinglePlunger Fuel Pump” and U.S. Pat. Nos. 7,401,594 and 7,124,738 (bothtitled “Damper Mechanism and High Pressure Fuel Pump”) describetechniques for internal damping of pressure pulsations in the lowpressure side of a high pressure fuel pump. Two metal diaphragms arejoined together around the circumference to envelope an internal volumeof pressurized gas (“double diaphragm”). One or two such doublediaphragm assemblies are situated in a damping chamber upstream of theinlet valve for the high pressure pumping piston.

The damper system of U.S. Pat. No. 7,124,738 has some inherentdrawbacks. Firstly, two convoluted diaphragms are welded to each otheron the outer periphery. This creates a condition where the weld strengthis compromised. Because the diaphragms are made from a thin material,the resulting weld bead is of a small cross-section and the assembly(with thin capsule shaped diaphragms) has minimal hoop strength andresistance to flexing at the weld. This was overcome according to the'738 patent by clamping the diaphragms with significant force inboard ofthe weld bead. Secondly, the damper will be overstressed and fail in theconvolutions if excess pressure pulsations are encountered due toactivation of the high pressure system overpressure relief valve. Whenthis relief valve is functioning, pressure pulsations of two to threetimes normal operation are encountered. Finally, when one of theflexible diaphragms does fail, the common volume of gas shared by thetwo diaphragms becomes filled with fluid, causing both diaphragms tobecome inoperable.

SUMMARY

It is an object of the present invention to provide a plurality ofsingle metal diaphragms that are configured into a small diaphragmassembly for a fuel inlet damper assembly or mechanism, but without theinherent problems associated with the use of a “double diaphragm”.

This is accomplished by joining two metal diaphragms to respective twosides of an imperforate central plate, thereby creating two closelyspaced diaphragm dampers, each acting upon its own gas volume.

In one aspect, the disclosure is directed to an inlet fuel pressuredamper mechanism for a high pressure fuel pump, comprising a centralmetal plate and two flexible metal diaphragms each separately joined torespective opposite faces of the central plate, thereby trapping twoseparate gas volumes between the central plate and each of the metaldiaphragms, whereby each diaphragm reacts to pressure pulsations bydeflecting independently against its respective gas volume.

Preferably, the diaphragm assembly has (a) a rigid, relatively thickcircular or polygonal central plate, (b) a first circular diaphragmhaving a rim portion sealingly secured as by welding to the plate and arelatively thin, flexible, convex portion projecting from one side ofthe plate and defining a first pressurized gas volume, and (c) a secondcircular diaphragm having a rim portion sealingly secured as by weldingto the plate and a relatively thin, flexible, convex portion projectingfrom the other side of the plate and defining an independent secondpressurized gas volume.

The central plate does not flex during normal operation and acts as astiffening member, thereby reducing the hoop stress and flexing of theweld. Additionally, the central plate can be configured with diaphragmstroke limiting features that reduce diaphragm flexing and stress whensubjected to excess pressure pulsations as can arise when the highpressure system overpressure relief valve operates.

In another aspect, the disclosure is directed to a pressure damperassembly for a fuel inlet passage in a single piston high pressure fuelpump, comprising an inlet fitting, a cover attached to the fitting andhaving a substantially cylindrical sidewall defining an internal chamberin fluid communication with the fuel inlet passage of the pump, and anopen bottom with a bottom edge sealingly attached to the pump. At leastone diaphragm assembly is supported within the damping chamber. Eachdiaphragm assembly includes first and second metal diaphragms havingdiaphragm rims sealingly secured to respective first and second sides ofa central plate and convex central regions spaced from the centralplate, thereby defining first and second independent closed gas volumesradially inward of a surrounding diaphragm assembly rim. In this way,feed fuel delivered to the pump through the inlet fitting flows throughthe damping chamber at a pressure that acts on the first and seconddiaphragms of each diaphragm assembly before entry into the inletpassage of the pump.

A base plate can optionally be provided to close the bottom of the coversuch that the damper unit is entirely independent of the pump except forattachment of the unit to the pump.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the invention will be described with reference to theaccompanying drawing, in which:

FIG. 1 is an overall schematic illustrating a fuel system for aninternal combustion engine;

FIG. 2 shows a first embodiment of the pressure damping diaphragmassembly of the present invention, which implements the function of thepressure damper of FIG. 1;

FIG. 3 shows a top view of the diaphragm assembly of FIG. 2;

FIG. 4 shows a second embodiment of the present invention;

FIG. 5 shows one possible installation of the damper of FIG. 4 withinthe pump described with respect to FIG. 1;

FIG. 6 shows a third embodiment of the present invention;

FIG. 7 shows a pair of diaphragm assemblies according to FIG. 3,configured as a damper assembly;

FIG. 8 shows the damper assembly of FIG. 7 installed on a pump as analternative embodiment to the configuration of FIG. 5; and

FIG. 9 shows another embodiment of a damper assembly according to theinvention.

DETAILED DESCRIPTION

FIG. 1 is an overall system schematic illustrating the fuel system foran internal combustion engine. The low-pressure pump 2 pressurizes fuelfrom the fuel tank 1, and delivers it to the high pressure pump housing3 through an inlet fitting. The fuel then passes through a pressuredamper including diaphragm assembly 4, and through a normally closedcontrol valve 5. Alternatively, the pressure damper could be upstream ofthe pump housing 3. A normally open control valve is also applicable tosuch a fuel system. The fuel is then drawn into the pumping chamber 10,where it is pressurized by the upward motion of the pumping piston 8 viathe engine camshaft 9. The control valve 5 is acted upon by the controlvalve spring 7 and solenoid 6 to control the quantity of fuel deliveredby the high pressure pump. This is accomplished by the accurate timingof the control valve closing relative to the pumping piston upwardtravel position. When the fuel is pressurized, it then travels throughthe outlet check valve 11, high pressure line 18, and into the commonrail 13 that feeds the engine fuel injectors 14. Because the injectors14 are fed from a common rail 13, injector timing is flexible. Desiredrail pressure is controlled by closed loop ECU 16 feedback and controlof the high pressure fuel output via the solenoid 6 and control valve 5compared to the rail pressure sensor 15 output signal to the ECU 16. Apressure relief valve 12 is required to protect the high pressure systemin case of a system malfunction. It is housed in a common fittingassembly 17, which also houses the outlet check valve 11. The pressurerelief valve can also be used to control the maximum system pressure toa predefined limit to protect other fuel system components.

FIG. 2 shows the first embodiment of the present invention, which isdirected to the diaphragm assembly 4 of the pressure damper of FIG. 1.Each single metal diaphragm 20 and 21 is welded at its outer peripheryby welds 22 and 23 to the surface of center plate 19, thereby trappingtwo separate gas volumes 25 and 26. The gas volumes can be at the samepressure, or each at its own pressure level (including vacuum), as setduring each of the welding operations. The three-part diaphragm assembly(diaphragm 20, diaphragm 21 and center plate 19) is situated in adamping chamber. The diaphragm assembly 4 defines two distinct andindependent diaphragms 20, 21 configured as a very space-efficient unit.Because the center plate is not acted upon by fluid pressure, andbecause it is shaped to be much stiffer in flexure and hoop than thediaphragms, the weld beads 22 and 23 see a low cyclic stress. Thereforethe mounting or support of the diaphragm assembly within the dampingchamber need not be designed to minimize stresses on the welds 22, 23.

Center plate 19 can be a flat plate, or can incorporate a series ofraised features 24 which limit the diaphragm deflection to a defineddistance ‘a’. The raised features can be designed to contact thediaphragm in one or more locations, i.e., the raised features can becircular or discrete dimples or the like. Cooperating feature 29 is anannular, internally directed groove on the diaphragms, aligned with andintended for contacting the raised features 24 on the center plate.Features 29 can likewise be discrete dimples or the like. The strokelimiting feature 24 can also be designed to work without the annulargroove 29. When distance ‘a’ is reduced to zero during operation (as canbe the case when the high pressure relief valve is in operation), theannular groove 29 of diaphragms 20 and/or 21 becomes supported by thecenter plate, minimizing any added stress in the diaphragms and allowingthe two diaphragms to survive. After the excessive pulsations, eachdiaphragm will return to normal function, operating without contactingthe center plate. Another advantage of the present invention is theadded benefit of a lower level of function if one diaphragm should fail.If diaphragm 20 should fail, and volume 25 fills with fluid, diaphragm20 will become inoperable. However, the diaphragm assembly 4 will stillfunction to a lesser degree because damper 21 and volume 26 will remainfunctional. The periphery or rim 27 of the center plate 19 extendsradially outside the welds 22, 23 and can be used to locate and securethe diaphragm assembly 4 without contact against any of the diaphragms20, 21 or welds 22, 23.

FIG. 3 shows the top view of the diaphragm assembly of FIG. 2. Althoughthe diaphragm assembly can be circular, the preferred embodiment asshown has a plurality of lobes 27 at a relatively larger radius from thecenter alternating with a plurality of flats 28 at a relatively smallerradius from the center. The diaphragm assembly is preferably supportedwithin the damping chamber by a fixture or retainer, at the lobes 27.The flats 28 allow fuel flow between the outer side of the upperdiaphragm 20 and the outer side of the lower diaphragm 21 when thediaphragm assembly 4 is situated in an infeed fuel path in the dampingchamber. The flow feature 28 could be almost any shape that allowsadequate flow area.

FIG. 4 shows a second embodiment 4′ of the present invention. In thisembodiment, the periphery of the circular diaphragms is at the sameradius as the periphery of the circular central plate, so mounting rim27 is eliminated. The mounting support of the diaphragm assembly can beover or adjacent to the welds 22′, 23′, and the flow features similar to28 of FIG. 3 can be incorporated into a mating component.

FIG. 5 shows one possible installation of the damper 4′ of FIG. 4.within the pump 3 described in FIG. 1. Item 31 is an inlet fitting ofthe high pressure pump. Item 30 is a cover defining the damping chamber,which is closed by the pump housing 35. Retainers 32, 33 and spacer 34locate and provide feed fuel flow to the pair of diaphragm assemblies4′. The retainer assembly 32, 33, and 34 is compressed between anoblique or horizontal portion of the cover 30 and a substantiallyhorizontal surface of the pump housing 35. Retainer 32 has a rim portion32 a that bears on and biases the top of the rim of the upper diaphragmassembly, retainer 33 has a rim portion 33 a that bears on and biasesthe bottom of the rim of the lower diaphragm assembly, spacer 34 has aradially outwardly directed rim portion 34 a that bears on and biasesthe bottom of the rim of the upper diaphragm assembly, and spacer 34 hasa radially outwardly directed rim portion 34 b that bears on and biasesthe top of the rim of the lower diaphragm assembly. The second retainer33 fits within and is laterally fixed in position by a recess 38 in thepump that is in fluid communication with the fuel inlet passage 37.

The upper retainer 32 has a convex upper portion 32 b including aplurality of cut outs or spaces 32 c and the lower retainer 33 also hasa convex lower portion 33 b including a plurality of cut out or spaces33 c. The spacer is substantially ring shape, with spaced apart holes 34c. The cutouts, spaces, and/or holes in the retainers and spacersprovide flow paths from all directions onto all four of the diaphragms.

When the bottom edge 30 a of the cover is welded to the top of the pump35, the compressed, biased condition of the rims of the diaphragmassemblies 4′ maintains the diaphragm assemblies in place within thedamping chamber. Inlet flow path 36 provides communication with the lowpressure pump 2 and outlet flow path 37 provides communication with thepump control valve.

FIG. 6 shows a third embodiment 4″ of the present invention, as avariation of the diaphragm assembly of FIG. 4. The welds 22″ and 23″ arenot located at the outer periphery of the diaphragms and center plate,but are instead through-welds on the diaphragm rims, which penetrateinto the rim of the center plate.

FIG. 7 shows a pair of diaphragm assemblies 4 according to FIG. 3,configured as a damper assembly 39, and FIG. 8 shows such damperassembly installed in a damping unit 40 on a pump as an alternative tothe configuration of FIG. 5.

The center plate 27 of each diaphragm assembly 4 has substantially flatupper and lower surfaces and a peripheral edge, with the upper and lowersurfaces extending two dimensionally with a maximum radial span S₁defined by the lobes 27 and a minimum radial span S₂ defined by flats28. The plate minimum span is at least equal to the radius R of thediaphragm rims, preferably greater, so the diaphragm rims are bonded tothe plate inside the minimum span. Each diaphragm assembly 4 a, 4 b issupported in the damping chamber 41 transversely to its plate 20, atportions of the upper and lower plate surfaces that are outside the rimsof the diaphragms, and each diaphragm assembly is laterally supported inthe damping chamber, at portions of the peripheral edge of the plate.Preferably, each diaphragm assembly is supported in the damping chambertransversely to the plate, at upper and lower surfaces of the lobes 27.The diaphragm assembly can be laterally supported in the dampingchamber, at the peripheral edges of the lobes, or as shown, at theperipheral edges of the flats 28. The upper and lower support can be ator include the weldments 22, 23 to the lobes. Whereas the diaphragmspreferably have a circular circumference, the plate can be any regulargeometric shape, such as a circle or polygon, and preferablysubstantially triangular with lobes at the corners.

In many embodiments the periphery of the diaphragms is smaller than theperiphery of the plate. The rim of each diaphragm is welded to theplate, such that symmetric portions of the plate extend radially outsidethe weld of the diaphragms to the plate. Support of the diaphragmassembly can be at the lobes, at radial positions that are optionallyoutside the rims of the diaphragms, on the periphery of the rims of thediaphragms outside the welds, over the welds, or inside the welds.

In the embodiment of FIG. 8, each of the upper and lower diaphragmassemblies 4 a, 4 b is supported within the damping chamber by aretainer assembly. A first retainer 42 of the retainer assembly has arim portion 42 a that bears on and biases the upper surface of one plate27, a second retainer 43 has a rim portion 43 a that bears on and biasesthe lower surface of the other plate, and a spacer 44 has an inwardlydirected portion 44 a that bears on and biases the lower surface of theone plate and another inwardly directed portion 44 b that bears on andbiases the upper surface the lower plate. The upper and lower retainershave outwardly angled tabs 45 that snap vertically into slots 46 formedbetween the spacer 44 and the flats 28. This holds the parts together asa unit 39 and restrains lateral displacement of the upper and lowerdiaphragm assemblies, respectively.

Another embodiment with the diaphragm assemblies of the type shown inFIG. 4 or 6 is shown in FIG. 9. The damper unit 47 is also attached tothe pump 35 as a separate system. As with the other embodiments, a pairof upper and lower diaphragm assemblies is supported within the dampingchamber 48 by a retainer assembly 49 that is somewhat similar to thatshown in FIG. 5. A first retainer 50 has a portion that bears on andbiases the top of the rim of the upper diaphragm assembly, a secondretainer 51 has a portion that bears on and biases the bottom of the rimof the lower diaphragm assembly, and a spacer 52 has a radiallyoutwardly directed portion that bears on and biases the bottom of therim of the upper diaphragm assembly, and another radially outwardlydirected rim portion that bears on and biases the top of the rim of thelower diaphragm assembly. The cover 53 retains a base plate 54 such thatthe damping chamber 48 is defined within the cover independently of thepump. When welded to the body, the cover 53 compresses the damperassembly 49 and thereby effectuates the biases among the first retainer,second retainer, spacer, and pair of diaphragm assemblies. The baseplate 51 has an exit port alignable with the fuel inlet passage 37 ofthe pump.

In FIG. 9, the upper and lower retainers 50, 51 have obliquely inwardlydirected prongs 56 that engage the outer edges of the spacer 52, therebycapturing and retaining the rims of the diaphragm assemblies. Anotherfeature of the embodiment shown in FIG. 9, is that the retainer assembly49 is radially restrained (i.e., substantially centered) within thecover 53, by contact between the retainers 50, 51 and the sidewall ofthe cover.

Alternatively, the retainer assembly 49 can be entirely self-supportingwithin the damping chamber, by a clamp or the like (not shown) providedbetween the base plate 54 and the first retainer 50 to compress the unitand thereby effectuate the biases among the first retainer, secondretainer, and coil spring between a pair of diaphragm assemblies,without using the installation force of the cover 53. In this and otherembodiments, the spacer 52 can alternatively comprise a coil spring thaturges the upper diaphragm assembly upward and the lower diaphragmassembly downward against respective upper and lower retainers.

The invention claimed is:
 1. A pressure damper assembly for a fuel inletpassage in a single piston high pressure fuel pump, comprising: an inletfitting; a cover attached to the fitting and having a substantiallycylindrical sidewall defining an internal damping chamber in fluidcommunication with the fuel inlet passage of the pump, and a bottomsealingly attached to the pump; a pair of upper and lower diaphragmassemblies supported within the damping chamber by a retainer assemblywherein each said diaphragm assembly includes first and second metaldiaphragms having diaphragm rims sealingly secured to respective firstand second sides of an imperforate central plate and convex centralregions spaced from the central plate, thereby defining first and secondindependent closed gas volumes radially inward of a surroundingdiaphragm assembly periphery; said retainer assembly includes a distinctfirst retainer having a portion that bears on and biases the peripheryof the upper diaphragm assembly, a distinct second retainer having aportion that bears on and biases the periphery of the lower diaphragmassembly, a distinct spacer between the first and second retainers,having a portion that bears on the periphery of the upper diaphragmassembly in opposition to the first retainer and another portion thatbears on the periphery of the lower diaphragm assembly in opposition tothe second retainer, with each of the first and second retainersincluding tabs that engage the spacer, said tabs being situated in slotsin the spacer; whereby feed fuel delivered to the pump through the inletfitting flows through said damping chamber at a pressure that acts onthe first and second diaphragms of each diaphragm assembly before entryinto the inlet passage of the pump.
 2. The damper assembly of claim 1,wherein the plate and diaphragms are circular, and the plate has alarger radius than the diaphragms; and the rim of each diaphragm iswelded to the plate, thereby defining a rim portion of the plate thatextends radially outside the weld of the diaphragms to the plate.
 3. Thedamper assembly of claim 1, wherein the plate and diaphragms arecircular, and have the same radius defining the circumference of thediaphragms and plate; and the rim of each diaphragm is welded to theplate radially inside of the circumference of the diaphragm.
 4. Thedamper assembly of claim 1, wherein the plate and diaphragms arecircular, and have the same radius defining the circumference of thediaphragms and plate; and the rim of each diaphragm is welded to theplate at the circumference of the diaphragm.
 5. The damper assembly ofclaim 1, wherein the first diaphragm is welded to one side of the plateand the second diaphragm is welded to the second side of the plate; andthe weld of the first diaphragm to the plate is independent of the weldof the second diaphragm to the plate.
 6. The damper assembly of claim 1,wherein the retainer assembly is compressed between a portion of thecover and a surface of the pump; said first retainer of the retainerassembly has a rim portion that bears on and biases the top of the rimof the upper diaphragm assembly; said second retainer of the retainerassembly has a rim portion that bears on and biases the bottom of therim of the lower diaphragm assembly; said spacer of the retainerassembly has a rim portion that bears on and biases the bottom of therim of the upper diaphragm assembly, and another rim portion that bearson and biases the top of the rim of the lower diaphragm assembly;whereby the compressed retainer assembly maintains the diaphragmassemblies in place within the damping chamber.
 7. The damper assemblyof claim 6, wherein the cover has an open bottom; and the secondretainer fits within and is laterally fixed in position by a recess inthe pump that is in fluid communication with the fuel inlet passage. 8.The damper assembly of claim 1, wherein the plate has substantially flatupper and lower surfaces and a peripheral edge, and the upper and lowersurfaces of the plate extend two dimensionally with different maximumand minimum spans; the plate minimum span is at least equal to thediameter of the diaphragm rims; the diaphragm rims are bonded to theplate inside the plate maximum span; and the damper assembly issupported in the damping chamber transversely to the plate, at portionsof the upper and lower plate surfaces that are outside the rims of thediaphragms.
 9. The damper assembly of claim 8, wherein the damperassembly is laterally supported in the damping chamber, at portions ofthe peripheral edge of the plate.
 10. The damper assembly of claim 8,wherein the plate has a plurality of lobes having the maximum span and aplurality of flats having the minimum span; and the damper assembly issupported in the damping chamber transversely to the plate, at upper andlower surfaces of the lobes.
 11. The damper assembly of claim 10,wherein the damper assembly is laterally supported in the dampingchamber, at the peripheral edge of the lobes.
 12. The damper assembly ofclaim 10, wherein the damper assembly is laterally supported in thedamping chamber, at the peripheral edge of the flats.
 13. The damperassembly of claim 1, wherein said first retainer of the retainerassembly has a rim portion that bears on and biases the upper surface ofthe plate of the upper diaphragm assembly; said second retainer of theretainer assembly has a rim portion that bears on and biases the lowersurface of the plate of the lower diaphragm assembly; said spacer of theretainer assembly has a portion that bears on and biases the lowersurface of the plate of the upper diaphragm assembly and another portionthat bears on and biases the upper surface of the lower plate of thelower diaphragm assembly.
 14. The damper assembly of claim 13, whereinthe tabs restrain lateral displacement of the upper and lower diaphragmassemblies, respectively.
 15. The damper assembly of claim 1, whereinthe cover has a base plate such that the damping chamber is definedwithin the cover independently of the pump; said unit is situatedentirely within said damping chamber; and the base plate has an exitport aligned with the fuel inlet passage of the pump.
 16. The damperassembly of claim 15, wherein the retainer assembly is compressedbetween a portion of the cover and the base plate.
 17. The damperassembly of claim 16, wherein the diaphragms assemblies are laterallyrestrained in the damping chamber by contact between the retainerassembly and the sidewall of the cover.
 18. The damper assembly of claim1, wherein the retainer assembly forms a self-supporting unit.
 19. Apressure damper assembly for a fuel pump, comprising: a pair of upperand lower diaphragm assemblies, each said diaphragm assembly includingfirst and second metal diaphragms having diaphragm rims sealinglysecured to respective first and second sides of an imperforate centralplate and convex central regions spaced from the central plate, therebydefining first and second independent closed gas volumes radially inwardof a surrounding diaphragm assembly periphery; a retainer assemblyincluding an upper retainer having a portion that bears on the peripheryof the upper diaphragm assembly, a lower retainer having a portion thatbears on the periphery of the lower diaphragm assembly, and a spacerbetween said upper and lower retainers having a portion that bears onthe periphery of the upper diaphragm assembly in opposition to the upperretainer and another portion that bears on the periphery of the lowerdiaphragm assembly in opposition to the lower retainer, with each of theupper and lower retainers including obliquely inwardly directed prongsthat engage outer edges of the spacer; whereby said upper and lowerdiaphragm assemblies, said upper and lower retainers and said spacer areconnected so that said upper and lower diaphragm assemblies, said upperand lower and said spacer are held together as a unit.
 20. The pressuredamper assembly of claim 19 wherein the prongs from the upper retainerextend obliquely downward and inward and the prongs from the lowerretainer extend obliquely upward and inward.
 21. The pressure damperassembly of claim 19 wherein said central plate includes raised featuresthat limit the deflection of one or both of said metal diaphragms.